I rather think of qw as quote whitespace since what it does is break up the subject string using whitespace as separators. So the string inside the qw() is broken into 3 pieces, not 3 words as in this case each resulting piece contains 2 words and some square brackets. See perlop under Quote and Quote-like operators.

The next think you need to know is that \Q and \E are allowing you to use the content of the variable $what as literal text without interpreting any special characters, which in this case are the square brackets. Have a look at perlre to see what [box] means inside a regular expression.

So in summary it is looking for the string [box] including the square brackets, withing the 3 separate strings in[box], out[box], and white[sox], and prints matches.

That's how I initially learned it as well, but perldoc.perl.org j ust says "quote a list of words" where you have to sort of liberally accept that a word is letters separated by whitespace. "Quote with whitespace" makes more sense to me than "quote whitespace", but it mostly just matters that it works as advertised.